In a case where a relatively large load is applied to a vehicle body panel that constructs a vehicle body, it is inevitably necessary to reinforce the vehicle body panel. Upon reinforcing the vehicle body panel in such a case, such a conventional technique as described in Patent Literature 1 has been used in general.
Patent Literature 1 discloses a structure for reinforcing a vehicle body panel, specifically, for reinforcing an underfoot floor panel on which feet of a passenger seated in a rear seat rest and to which a relatively large load is exerted. In the structure as described in Patent Literature 1, the floor panel is provided with a forward-and-rearward bead that extends over a substantially entire length in a forward-and-rearward direction of the vehicle such that the floor panel has a wave form in cross-section to thereby contemplate the reinforcement.
However, in the typical structure for reinforcing a vehicle body panel as described in Patent Literature 1, it is difficult to ensure a sufficient bending strength in a direction of the wave form of the vehicle body panel, and the vehicle body panel tends to be readily deformed upon receiving a bending load in the direction of the wave form. That is, there exists such a directional dependency of degree of reinforcement of the vehicle body panel that a reaction force to a load in a specific direction is large, but a reaction force to a load in another direction is small.
For the above reason, when a thickness of the vehicle body panel is determined, it is necessary to increase a thickness of the panel in order to obtain a predetermined strength of the panel even against a bending load in a direction in which a degree of reinforcement is low. As a result, there occur problems such as an increase in weight and an increase in cost.
In addition, in the general technique of providing the bead extending over the substantially entire length of the vehicle body panel, it is necessary to prepare a panel having a size which is increased by a shrinkage tolerance of the bead in a direction of arrangement of the bead and adjust the size of the panel upon forming the bead so as to comply with the vehicle body panel having a predetermined size. Since the panel having a size larger than that of the vehicle body panel, there will inevitably occur problems such as increase in weight and increase in cost.
Further, in order to reinforce a vehicle body panel, it is also considered that an surface area of the vehicle body panel per se is reduced by adding a fore-and-aft member extending in a forward-and-rearward direction of the vehicle body and a lateral member extending in a width direction of the vehicle body.
However, such addition of the fore-and-aft member and the lateral member causes not only the problems such as increase in weight and increase in cost but also the following problems due to projection of these members toward one side of the vehicle body panel in a thickness direction of the vehicle body panel.
In a case where the vehicle body floor is an underfoot floor panel on which feet of a passenger seated in a rear seat rest and which is required to bear a relatively large load, the problems are explained in detail as follows.
In a case where the fore-and-aft member and the lateral member are additionally provided on an upper side of the underfoot floor panel, an underfoot level (a heel point) at which the passenger's feet rest is raised by heights of these members so that the passenger has a strained feeling upon seating in and leaving the rear seat, or such an uncomfortable feeling that the feet are positioned at an excessively high level when the passenger is in the seated state.
In contrast, in a case where the fore-and-aft member and the lateral member are added to a lower side of the underfoot floor panel, these members are downwardly projected by heights of these members from the underfoot floor panel. Accordingly, in a case where a large-sized battery pack is installed beneath the underfoot floor panel in an electric vehicle such as an electric vehicle and a hybrid vehicle, the battery must be downsized by an amount of projection of the members so that a capacity of the battery is sacrificed.